When must a pilot judge sufficient separation exists before landing?

A pilot must judge that sufficient separation exists before landing primarily when air traffic control (ATC) instructs them to land after another aircraft. This is essential for maintaining safety and preventing collisions on the runway. ATC is responsible for managing air traffic and providing instructions to pilots about when to land, taking into account the distance and speed of other aircraft in the vicinity.

The control tower closely monitors the landing sequence and will issue clearance once they determine that the runway is safe for the landing aircraft. The responsibility lies with the pilot to adhere to this instruction and to ensure that sufficient separation exists as determined by ATC's guidance.

In other scenarios, such as choosing to land at any time independently or during adverse weather conditions, pilots are also responsible for assessing whether conditions allow for a safe landing. However, these judgments are typically made outside of ATC instruction, making option B the most applicable scenario for specific separation judging. Congested airports may require extra caution, but it is still ATC that provides the necessary landing clearance and manages aircraft spacing.